1. Field of the Invention
The present invention relates to exposure processing in an exposure system in which a plurality of laser sources are located at dispersed sites.
2. Description of the Background Art
As CTP (Computer To Plate) systems for direct imaging on printing plates, those using laser diodes (LDs) in an exposure head (a light source) have been conventionally widely known. Such CTP systems have widely adopted multi-beam exposure methods in which exposure is performed with simultaneous light emission from a plurality of laser diodes.
For example, CTP systems using spiral exposure methods are known, which systems include an exposure head (a multi-channel optical head) including a plurality of laser diodes aligned in one direction and a lens focusing laser beams emitted from those plurality of laser diodes to form an image in a position of exposure, and in which systems with rotation of a drum (printing drum) having a printing plate mounted thereto, laser beams are applied from the laser diodes to expose approximately the entire surface of the printing plate while the exposure head moves in the axial direction of the drum (see, for example, Japanese Patent Application Laid-open No. 2000-43317).
Other CTP systems are also known, which systems include an exposure head with a plurality of laser diodes located at dispersed sites at regular intervals, and in which systems with rotation of a drum (printing drum) having a printing plate mounted thereto, laser beams are applied from the laser diodes directly onto the printing plate while the exposure head moves in the axial direction of the drum by a distance equivalent to the interval between the laser diodes, so as to simultaneously expose a plurality of partial areas and ultimately to expose approximately the entire surface of the printing plate (see, for example, Japanese Patent Application Laid-open No. 2003-89180).
JP-A-2000-43317 discloses the technique in which laser diodes are grouped into two parts, the first half and the second half, and when any of the laser diodes in either the first half or the second half stops emitting light (illuminating) due to its breakage or the like, exposure is performed by use of only the laser diodes in the other half, whereby exposure can be performed continuously without halting the system until the next change of the laser diodes.
However, this technique has a drawback that exposure cannot be performed in the case where the first half and the second half both include a non-light emitting laser diode.
Besides, this technique is, in the first place, aimed at the CTP systems using spiral exposure methods as disclosed in JP-A-2000-43317. Since the CTP systems including a plurality of laser diodes located at dispersed sites, as disclosed in JP-A-2003-89180, have a limited moving range of individual laser diodes during exposure, it is impossible to adopt the technique as disclosed in JP-A-2000-43317 that the laser diodes in the first half, for example, can expose areas that should originally be exposed by the laser diodes in the second half.